1. Field of the Invention
The present invention relates to a centralized supervisory system and, more particularly, to a centralized supervisory system in an automatic switching system comprising of a plurality of remote offices and a central office. In the automatic switching system to which the present invention pertains, each remote office has a remote-office time-division switching system centrally supervised by the central office. The central office has a central office time-division switching system connected through pulse-code modulation (PCM) multichannel lines to the remote-office time-division switching systems.
2. Description of the Prior Art
Conventionally, space-division switching systems have been employed in the remote offices and in the central office. In one such conventional system, the remote-office space-division switching systems and the central-office space-division switching system have been connected by the number of wired transmission lines corresponding to the volume of calls carried. That is, the transmission lines have not been multichannel, but single channel or single telephone subscriber lines.
In another such conventional system, the remote offices and the central office have been connected by a single analog multichannel line, but even here the output signals of each space-division switching system have been first provided on a number of separate transmission lines and then multiplexed on the single analog multichannel line and/or the signals on the single analog multichannel line have been divided from each other and then supplied through a number of transmission lines to the space-division switching systems. Therefore, even though single analog multichannels are employed, the remote offices and the central office can still be said to have been connected by a number of transmission lines each corresponding to single channels.
In the conventional systems mentioned above where the remote offices and the central office are connected by a number of transmission lines each corresponding to single channels, it was relatively easy to use some of the transmission lines especially for transferring supervisory data from the remote offices to the central office.
Recent technology, however, has led to the development of time-division switching systems in place of space-division switching systems. When time-division switching systems are employed in both the remote offices and the central office, the connections therebetween are realized by a single PCM multichannel line for time-division multiplexed lines. The use of PCM multichannels, however, makes it difficult to use some of the channels especially for transmitting data from the remote offices to the central office.
Instead of using some of the channels especially for transmitting supervisory data, separate transmission lines may be provided for supervisory data, independent from the PCM multichannel line. It is obvious, however, that the provision of separate transmission lines would incur unwarranted additional expenses.